Bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) provide a low-cost, large-area, flexible, light-weight, clean and quiet alternative energy source for a variety of indoor and outdoor applications. However, the relatively low power conversion efficiency (PCE) and relatively short operational lifetimes of such organic photovoltaic cells is an impediment to their mainstream adoption.
To overcome the deficiencies of OPVs, and to achieve power conversion efficiencies in excess of 10%, bulk heterojunction (BHJ) materials capable of generating high short-circuit current densities (Jsc) and large open-circuit voltages (Voc) have been investigated. One approach that has been pursued to increase the short-circuit current density (Jsc) and the open-circuit voltage (Voc) of organic photovoltaic (OPV) cells is to develop low-band-gap semiconducting polymers with deeper highest occupied molecular orbital (HOMO) energies. The pathway to low-band-gap semiconducting polymers with deeper HOMO energies is well established, and the OPV cell fabricated using these semiconducting polymers have yielded high power conversion efficiencies (PCEs).
Still another approach to increasing the short-circuit current density (Jsc) and the open-circuit voltage (Voc) of BHJ OPV cells is to develop electron acceptor materials that absorb more light and with higher lower un-occupied molecular orbital (LUMO) energies. However, the development of such electron acceptor materials lags behind the rate of innovation of organic photovoltaic (OPV) cells.
The construction of molecular structures of a-conjugated molecules via self-assembly has been recognized as an important approach to manipulate their optical and electronic properties to create “molecular” electronics. Specific device configuration often requires explicit molecular arrangements of these electronically active molecular segments. For example, to achieve high power conversion efficiency (PCE) in OPV cells, a bi-continuous interpenetrating structure/morphology with a large interface between the electron donor (D) and electron acceptor (A) and separate channels for electron and hole transport there between is highly desired. In order to keep the electron and hole channels separated, so to retain long charge separation lifetimes, the “double channel” structure has been investigated due to its enhancement in device operating performance. This “double channel” structure typically includes a p-type conjugated polymer electron donor backbone grafted by n-type electron acceptor moieties, such as fullerene (C60). However, the traditional grafting approach used to attach the electron acceptor (A) and the electron donor (D) molecules lacks the precise control over both primary chemical and secondary physical structures, and as such, the operating performance of OPV cells formed thereby is compromised.
Thus, while using self-assembly to construct molecular assemblies of these π-conjugated molecules has been recognized as an important approach to manipulate their properties to create “molecular” electronics with dimensions typically at 5˜100 nm length scale, specific device configurations often require explicit molecular structures. For example, to achieve bulk heterojunction (BHJ) organic photovoltaic (OPV) cells having high power conversion efficiency (PCE), a bi-continuous network structure with a large electron donor/acceptor interface and separate, independent channel structures for electron and hole transport (i.e. ambipolar structure) is highly desired. While attempts at forming a double-channel polymer with a p-type polymer backbone and n-type electron acceptor moieties have been made, a well-defined, physical “double channel” structure with ambipolar electron/hole transporting properties has yet to be achieved.
Therefore, there is a need for a polymer that is conjugated with fullerene to form an active area of a organic photovoltaic (OPV) cell having double channels for separate, independent electron and hole transport. In addition, there is a need for an electron donor, such as conjugated polymers, conjugated oligomers, and conjugated small molecules, that are conjugated with other organic molecules, such as fullerenes and fullerene derivatives to form organic photovoltaic (OPV) cells having dual channels for separate, independent electron and hole transport. Still yet there is a need for an organic photovoltaic (OPV) cell having an active area formed of a thermally stable and processible discotic liquid crystal (LCs) of C60-Por dyad with a self-assembled molecular “double channel” structure. Moreover, there is a need for an organic photovoltaic (OPV) cell having an active area formed of a polymer conjugated with an electron acceptor that are linked together by an ambipolar structure, so as to provide a double channel point of attachment to allow the separate and independent transport of electrons and holes via the channels.